The primary objectives of this project are twofold; first, to follow a cohort of twin pairs who have previously participated in a three year study of the effects of calcium supplementation on skeletal growth to determine whether the preliminary observation of increased cortical bone density is maintained over time and second, to provide for an in-depth analysis of the extensive data collected during the first five years of study using this cohort. The calcium supplementation portion of this study was designed to take advantage of the unique genetic and environmental relationships present in the families of young identical twins. This co-twin control study design has been shown to be 2 to 14 times more efficient than the randomized complete block design in previous investigations. In the initial clinical trial school-aged identical twins were recruited for a three-year study of the effects on skeletal growth of 1000 mg supplemental calcium per day. Radial bone mass was determined at baseline, 6 months and yearly thereafter. Bone mass in the hip and spine were determined at baseline and at termination of supplementation. Assessments of physical activity and dietary intake were incorporated into the study design. Detailed anthropometric measurements were obtained at each clinic visit. At the termination of the supplementation period a detailed panel of serum and urine markers of bone metabolism were obtained. The goal of the present proposal is to continue to follow all twin pairs who complete the three-year study (N=47 pairs) on an annual basis for an additional four years. The children will have radial, spine and hip bone mass measured at each clinic visit. In addition estimates of total body calcium estimated by dual energy X-ray absorptiometry (DEXA) methodology will be obtained. Anthropometric measurements and dietary and physical activity assessments will also be obtained at each clinic visit. These are potential covariates related to bone mass and provide insight in the interpretation of the results. In summary, continued follow-up of the participants of the earlier study is important in the interpretation of the effects of calcium supplementation in children. Documentation of beneficial effects and elucidation of covariates have important implications for reduction of morbidity associated with osteoporosis later in life.